The importance of determining the efficiency of an analytical procedure for pesticide residue levels has long been recognized by residue chemists. Biochemists investigating the metabolic pathways of radiolabeled pesticides in laboratory animals, however, frequently report extraction efficiencies for certain solvents and solvent systems as applied to the recoveries of radioactive materials from certain tissues of interest. The continuing development and evaluation of a microanalytical method for the determination of pesticides and their metabolites in human and animal tissues prompted an investigation of the absolute recovery of these compounds. Dieldrin and p,p'-DDT were chosen for study. Experiments were conducted to provide information concerning (a) the absolute recovery of radiolabeled materials from the tissues examined and (b) the percentage of radioactivity in the tissue extract being due to free or gas chromatographable pesticide. A comparative study of the recovery of unlabeled pesticides from human tissue samples was also undertaken to validly relate the recovery of these materials using several solvents and solvent systems.